Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability

Abstract

The electronic packaging industry has been using electroless Ni͑P͒ / immersion Au as bonding pads for solder joints. Because of the persistence of the black pad defect, which is due to cracks in the pad surface, the industry is looking for a replacement of the Ni͑P͒ plating. Several Cu-based candidates have been suggested, but most of them will lead to the direct contact of solder with Cu in soldering. The fast reaction of solder with Cu, especially during solid state aging, may be a concern for the solder joint reliability if the package will be used in a high temperature environment and is highly stressed. In this work, the reaction of eutectic SnPb solder with electrodeposited laminate Cu is studied. Emphasis is given to the evolution of the microstructure in the interfacial region during solid state aging and its effect on solder joint reliability. A large number of Kirkendall voids were observed at the interface between Cu 3 Sn and Cu. The void formation resulted in weak bonding between solder and Cu and led to brittle fracture at the interface in the ball shear and pull tests. The experimental results indicate that a barrier for Cu diffusion may be needed between the solder and the type of Cu used in the test vehicle for the packages that will experience high temperature ͑Ͼ100°C͒ and high stress.

Formation Mechanism of Porous Cu3Sn Intermetallic Compounds by High Current Stressing at High Temperatures in Low-Bump-Height Solder Joints

“…Previously, the Kirkendall void formation mechanism in Cu3Sn was proposed [17]. When the layer-type Cu6Sn5 decomposes into Cu3Sn, it will release 3 Sn atoms: So far, we have observed that the porous-type Cu 3 Sn IMCs can form during high current stressing at high temperatures and the effect of side wall wetting is obvious.…”

“…Previously, the Kirkendall void formation mechanism in Cu 3 Sn was proposed [17]. When the layer-type Cu 6 Sn 5 decomposes into Cu 3 Sn, it will release 3 Sn atoms:…”

Mechanism and Prevention of Spontaneous Tin Whisker Growth

“…However, we must recall that in some cases of interdiffusion in bulk diffusion couples, vacancy may not be in equilibrium everywhere in the sample, so very often Kirkendall void formation has been found due to the existence of excess vacancies. 28) For the fixed volume of V in the finish as considered in Fig. 5, to absorb the added atomic volume due to the indiffusion of Cu, we must be able to add lattice sites in the fixed volume.…”

Microstructural and mechanical analysis on Cu–Sn intermetallic micro-joints under isothermal condition

“…For instance, Kirkendall voids are normally found in Cu 3 Sn layer or near the interface between Cu 3 Sn and plated Cu substrate after aging or during the service period [14][15][16], which will strongly impact the reliability of the IMC interconnections.…”